Let’s be honest, there’s something about Nazis that makes them impossible to ignore. Those boots, those salutes, that deeply creepy goosestep. And then of course there’s the small matter of their unspeakable evil. Murdering 11 million people—six million of them for their religion alone—makes you unique among history’s monster states. But what about the individual monsters themselves?

The architects of the Holocaust long go achieved a sort of undifferentiated, single-name infamy—Eichmann, Himmler, Goering, Goebbels—all of them evil, all accursed, all deservedly so. But there’s also the fact that most of them were family men, strivers climbing the career ladder and bringing home paychecks to a wife and children whom they told themselves they loved—and they probably did.

It’s that duality—the man who can dote on a cherished handful, then go to the work murdering millions of others—that has always mystified us the most. The just-released cache of correspondence between Heinrich Himmler—the Reichsführer responsible for building the concentration camps—and his wife and daughter, has raised the mystery of this aspect of human behavior anew. Coincidentally, on the same day the news of the Himmler letters went wide, investigators at SISSA-International School for Advanced Studies in Trieste, Italy, have released the results of an experiment that may help explain it.

The sound-bites from the Himmler letters are jaw-dropping. There are the 150 tulips he sent his wife Margarete and their daughter from Holland, which his Nazis had only two years earlier terrorized and conquered. “Striped, jagged, two-color, one-color. The kind you cannot find [in Germany],” he wrote attentively. There are his regular updates on his important work, expressed with a chilling pride and nonchalance: “From Saturday until Tuesday I’ll be in a killing field to test new and interesting methods of shooting.” And there’s his affectionate farewell as he embarks on a business trip to a place that has become synonymous with the very essence of evil: “I’m off to Auschwitz. Kisses, Yours, Heini.”

Ethicists, morality researchers, historians and more have done as good a job as possible explaining how people who manifestly care for their own families can so easily slaughter members of other people’s. It begins by dehumanizing the group to be eradicated so that the act feels more like pest control than genocide. And so the Nazis depicted the Jews as rats in their propaganda films and the Hutus labeled the Tutsis “cockroaches” during the 1994 Rwanda genocide. Even history’s good guys—the Allied nations in the Second World War, for example—are capable of that. Hitler and Mussolini were caricatured as cartoonishly evil in American propaganda posters. The Japanese, however, were drawn as yellow-faced, fang-toothed and claw-handed—when they weren’t simply depicted as monkeys. (Continued below gallery)

But the Allies weren’t looking to exterminate all of the Japanese, Italians or Germans, while the Nazis had just that fate in mind for the Jews and other undesirables. In this, Himmler and his wife were aided by another tool of the actively evil: mutual validation. Humans do appear to have an innate sense of right and wrong—what researchers call “moral grammar”—and it’s with us from birth. Hatred and bigotry require that that impulse be beaten back, and cultural reinforcement helps. There’s a reason American racism is more common in the South which, even now, has more pockets that serve as echo chambers for bigotry than the North does. The more social opprobrium the racist feels, the faster hatred can be choked off. The Himmlers pumped plenty of fresh oxygen to their mutual antisemitism.

As early as 1928, Heinrich wrote to Margarete, who was trying to sell her share of a health clinic to a Jewish co-owner of the same building: “You have to fight with these wretched Jews because of the money. Don’t get frustrated over the Jews good lady—if only I could help you.”

A decade later, after Kristallnacht—the Nazi-sponsored looting and destruction of Jewish homes, shops and synagogues—Margarete wrote to Himmler, “This thing with the Jews. When will this scum leave us so that we can lead a happy life?”

The Himmlers even seemed to revel in a shared sense of what they saw as their impetuous wickedness. “I am so lucky to have a bad husband who loves his bad wife,” she wrote. Himmler responded, “There’s nothing other than ‘revenge.’ Forever.” Margarete then upped the evil ante. “My black soul is thinking about the most impossible things.” In a measure of the slipperiness of language, the German word Margarete used in describing them both as “bad” was “böse,” which can also mean naughty, misbehaved or, less commonly, evil. The U.K.’s Daily Mail, in its reports on the letters, used the “bad” translation. The Israeli newspaper Yediot Aharonot went for the full “evil.” Either way, the Himmlers hardly come out looking good.

The new Italian study explains—at least a little—the roots of their awfulness by looking at how the brain processes individuals versus groups. As long ago as 1984, neuroscientists began documenting the fact that people suffering from brain damage or dementia may have trouble categorizing nouns that belong in the category of either animate or inanimate things. Often, they did much better with one category—reliably identifying plants, animals and humans as animate, say—while struggling with the inanimate. Since damage can be particular to certain parts of the brain, this suggested that the two concepts are also processed in certain regions.

In the new study, the Italian researchers recruited a group of 21 subjects with varying stages of either fronto-temporal dementia or Alzheimer’s type dementia—which are specific to different areas of the brain—as well as a group of healthy control studies. The subjects were asked to make distinctions among three categories: the nonliving (including beds, forks, windows and hats), the living (including monkeys, onions, butterflies and horses) and groups (including gypsies, Jews, criminals and the disabled—all Nazi targets).

Across the board, the subjects with some form of dementia performed less well than the healthy, but there was no uniformity in the particular categories on which they did well or poorly. Without brain scans, it’s impossible to know which portions of the brain were responsible for the deficits, but one patient who had trouble with social categories was already known to have frontal temporal dementia, with the damage localized to the left hemisphere, so perhaps that is the spot that plays the biggest role.

None of this is remotely to suggest that the Himmlers—nor any of the other authors or enablers of the Holocaust—either suffered from brain damage or could blame their hate on physiological functions beyond their control. Instead, it seems the brain is, for better or worse, what one morality researcher calls a “meaning-making machine” and that it’s hardwired to make all manner of value judgments, including judgments concerning whole groups of people. Through circumstances, surroundings and our own disreputable choices, we may sometimes make hateful judgments. The Himmlers and their ilk were history’s grotesqueries. But all of us, like it or not, have the same brains—and the same capabilities.

The original version of this story identified the Italian institution that conducted the new research as the International School of Advanced Studies. The correct name is SISSA-International School for Advanced Studies.